The present invention relates generally to the field of electrostatic deposition of fine non-conductive particles onto a conductive substrate, and more particularly to such deposition onto a moving elongate filament on a high speed continuous basis.
Typically, filaments such as wires are coated with solvent borne organic materials for decorative or functional purposes. Apparatus for this application are described in the literature, a typical arrangement being described in U.S. Pat. No. 4,022,933. The use of solvents in these coating systems poses two difficulties. One is the organic emissions which have to be incinerated or catalytically oxidized to comply with emissions standards. The other is the cost of the solvents lost during the process.
The above referenced patent describes a system for coating wire using high solids chemistry in which the percent of solvent usage is reduced by perhaps as much as one-half over prior technology. However, such high solids coatings tend to have rheology problems during application due to their highly viscous state. Preheating of the coating material is generally required prior to application. Even so, the high viscosity can result in bare spots or misses in coverage of the substrate, and in another defect known as candle waxing or roping which is a longitudinally occurring radial variation in coating thickness.
An alternative approach to coating wire using powder chemistries offers benefits in many areas. Powder coating involves virtually no solvent, so emission standards can be met without expensive afterburners. Additionally, powder coatings can be formulated with higher molecular weights than liquid coatings which helps to produce tougher coatings, with generally greater resistance to environmental deterioration. Furthermore, if electrostatic application of the powder is used, bare spots and local irregularities, such as the candle waxing, can be minimized. U.S. Pat. No. 3,019,126 details both an electrostatic and non-electrostatic means for coating wire, with a fluidized bed as the central element.